Add `shm_open`/`shm_unlink` (POSIX shared memory) #3276

Job	Run time
Check nightly-only targets (nightly)	24s
Check selected Tier 3 platforms (nightly)	51s
Check (stable)	3m 33s
Check --no-default-features (nightly)	1m 9s
Check (nightly)	3m 36s
Rustfmt	20s
Check (1.63)	3m 50s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 32s
Test use-libc (ubuntu)	1m 14s
Test (ubuntu)	1m 9s
Test use-libc (i686-linux)	1m 28s
Test (ubuntu-20.04)	1m 1s
Test use-libc (aarch64-linux)	1m 41s
Test (i686-linux)	1m 11s
Test use-libc (powerpc64le-linux)	1m 35s
Test (aarch64-linux)	1m 10s
Test use-libc (riscv64-linux)	2m 7s
Test (powerpc64le-linux)	1m 10s
Test use-libc (arm-linux)	1m 38s
Test (riscv64-linux)	1m 8s
Test (s390x-linux)	1m 7s
Test (arm-linux)	1m 11s
Test (ubuntu-stable)	1m 8s
Test (i686-linux-stable)	1m 10s
Test (aarch64-linux-stable)	1m 11s
Test (riscv64-linux-stable)	1m 10s
Test (s390x-linux-stable)	1m 10s
Test (powerpc64le-linux-stable)	1m 10s
Test (arm-linux-stable)	1m 11s
Test (ubuntu-1.63)	1m 8s
Test (i686-linux-1.63)	1m 11s
Test (aarch64-linux-1.63)	1m 12s
Test (riscv64-linux-1.63)	1m 9s
Test (s390x-linux-1.63)	1m 10s
Test (powerpc64le-linux-1.63)	1m 10s
Test (arm-linux-1.63)	1m 8s
Test (macos-latest)	54s
Test (macos-11)	50s
Test (windows)	1m 41s
Test (windows-2019)	57s
	54m 45s

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